As fluid machines for feeding fluids under pressure, rotary-type (turbo-type) pumps such as axial flow pumps, mixed flow pumps or centrifugal pumps, and reciprocating-type (displacement-type) pumps such as plunger pumps are known in the art. The former type (turbo-type) of pump generally has pump characteristics for operating suitably in a low fluid-head/high flow-rate operating range in which the specific speed is high. On the other hand, the latter type (displacement-type) of pump has the pump characteristics for operating suitably in a high fluid-head/low flow-rate operating range in which the specific speed is extremely low. A vortex pump (cascade pump) is known as a type of pump which can operate in an intermediate operating range (the specific speed is about 30) between the operating range of the turbo-type pump and the operating range of the displacement-type pump.
In Japanese Patent Publication No. 3884880 and Japanese Laid-open Patent Publication Nos. 2003-13898 and 2004-132209 (Patent Documents 1 through 3), the present inventors have proposed turbomachines having a structure wherein numerous shallow grooves for restricting prerotation of recirculating flow are formed in the direction of the pressure gradient on the inner wall surface of a casing in order to prevent the instability characteristics, which is so-called “rising unstable head curve characteristics” or “rising head curve characteristics” and which is peculiar to the turbo-type pumps. This groove is known in this technical field as “J groove.” In the turbomachines described in Patent Documents 1 through 3 above, the prerotation flow is restricted by an extremely simple structure, in which the grooves are merely formed on the wall surface of the casing for suppressing various abnormal phenomena of fluid flow.
The inventors have also confirmed a phenomenon in that short grooves or depressions locally formed in the outer circumferential region of the back surface of the impeller (rotor wheel) of a centrifugal pump cause the fluid, which is discharged outward from the impeller, to flow back into the grooves. Such short grooves in the outer circumferential region of the back surface can be used as effective means for eliminating the aforementioned instability characteristics.
A centrifugal pump having an impeller, in which a small number of grooves are formed in its peripheral region, is disclosed in Japanese Laid-open Patent Publication No. 2002-227795. FIG. 18(A) is a schematic cross-sectional view showing the structure of this centrifugal pump. Grooves 102 of an impeller 101 are formed in its peripheral region 104. The centrifugal pump has a suction port 105 at its radially center part. The grooves 102 extend radially inward from an outer circumferential edge 103 of the impeller 101, but they do not reach the suction port 105. A flow rate restricting part 106 is formed between the grooves 102 and the suction port 105. In the flow rate restricting part 106, a stationary wall surface 107 of the pump casing is in the close proximity of a circular surface 108 of the impeller. The impeller 101 rotates about an axis X-X, so that a fluid “a” to be fed under pressure is forcibly pumped.
A centrifugal pump also having an impeller with a small number of grooves formed thereon is disclosed in Japanese Laid-open Patent Publication No. 2004-353564. FIG. 18(B) is a schematic cross-sectional view showing the structure of this centrifugal pump. The centrifugal pump has a suction port 115 at its radially center part. A small number of grooves 112 extend from an area of the suction port 115 to an outer circumferential edge 113 of the impeller 111. Numerous spiral grooves 119 constituting a dynamic bearing are formed in a peripheral region 114 of the impeller 111. The depth h of each of the spiral grooves 119 is about 10 to 100 μm. A side wall surface 117 of the pump casing is in the close proximity of a circular surface 118 of the impeller. The dimension v of a gap formed between the side wall surface 117 and the circular surface 118 is also about 10 to 100 μm.    Patent Document 1: Japanese Patent Publication No. 3884880    Patent Document 2: Japanese Laid-open Patent Publication No. 2003-13898    Patent Document 3: Japanese Laid-open Patent Publication No. 2004-132209    Patent Document 4: Japanese Laid-open Patent Publication No. 2002-227795    Patent Document 5: Japanese Laid-open Patent Publication No. 2004-353564